The present invention relates to a piezoelectric vibrator. More specifically, the present invention relates to a piezoelectric vibrator utilizing an expansion mode vibration or a breath mode vibration.
2. Description of the Prior Art
FIGS. 1 to 3 are views showing different examples of a square plate type vibrator utilizing a conventional expansion mode vibration in which the present invention can be advantageously employed. Such a piezoelectric vibrator 1 comprises a piezoelectric plate 11 of such as PZT, an electrode 12 formed on one main surface thereof, and an electrode 13 formed on the other main surface thereof. The vibrator shown in FIG. 2 is an example wherein the electrode 14 formed on the other main surface of the piezoelectric plate 11 is formed as a partial electrode i.e. an electrode formed only on a portion of the surface of the plate. The example shown in FIG. 3 is a so-called three-terminal type vibrator wherein the electrode 15 formed on the other main surface of the piezoelectric plate 11 is formed as a dot electrode and the electrode 16 formed on the same other main surface of the piezoelectric plate 11 is formed as a ring electrode. Since the principle of an operation of these piezoelectric vibrators is well-known to those skilled in the art, a more detailed description thereof will be omitted.
FIG. 4 is a graph showing a frequency characteristic of the FIG. 3 vibrator used in an intermediate frequency circuit of an AM band radio receiver, for example, as an example of a square plate type piezoelectric vibrator utilizing a conventional expansion mode vibration. As seen from the graph, the conventional example exhibits a large spurious vibration such as the third harmonic or the fifth harmonic of the fundamental vibration frequency. This spurious is a problem. For example, the third harmonic occurs as shown in FIG. 5. More specifically, such harmonic occurs, because a standing wave of the frequency three times the fundamental vibration frequency is superposed in the expansion direction of the piezoelectric plate 11. Therefore, as an approach for suppressing such spurious vibrations, an improvement for precluding such a standing wave by providing the electrode as partial electrodes 12a and 13a, as shown in FIG. 6, has been known. More specifically, by using the partial electrodes 12a and 13a, as shown in FIG. 6, the surface electric charge at the hatched portion in the surface electric charge distribution in FIG. 6 is canceled, whereby the vibration of the third harmonic is suppressed. By properly changing the geometry of the partial electrodes, the fifth harmonic and seventh harmonic or the edge mode vibration spurious vibration or the like can be suppressed. However, this approach merely suppresses only one intended spurious vibration (such as only the third harmonic) and cannot suppress several various spurious vibrations at the same time. Furthermore, in view of other requirements in the characteristic, it could happen that an electrode arrangement cannot be determined in consideration of only such spurious suppression.